This invention relates to backflow prevention devices and, particularly, to improved pressure differential relief valves or dump valves for use in reduced pressure principle backflow preventers.
Reduced pressure principle backflow preventers are known for use on direct connections where back pressure or back siphonage could present a health hazard. Typically, these devices include serially interconnected first and second check valves with a gate valve at the inlet side of the first check valve and another gate valve at the outlet side of the second check valve. The devices also include a diaphragm operated pressure differential relief valve connected to a zone intermediate the outlet of the first check valve and the inlet of the second check valve. This relief valve operates on the basis of preselected pressure differential between the intermediate zone and the inlet side of the first check valve as sensed on the opposite sides of the diaphragm in the relief valve. If, for any reason, the pressure differential should drop below the preselected value (for example, because of a malfunction of either check valve during periods of back pressure or back siphonage), the relief valve opens to discharge any backflowing contaminants to atmosphere and to maintain the proper pressure differential thereby protecting the potable water supplies.
In the design and construction of these backflow prevention devices, it has been a continuing problem to provide products with operational characteristics which comply with increasingly stringent industry and governmental standards. For example, in order to comply with presently prevailing performance standards, the relief valve in a reduced pressure backflow preventer must open when differential pressure across the first check valve drops below 2.0 psi and must reclose when the differential pressure increases to 3.0. Optimally, the relief valve should operate with a flat characteristic curve between 2.0 and 3.0 psi differential at all hydrostatic pressures and, most preferably, the valve's differential characteristic operating curve will be sufficiently flat so that it will operate reliably between 2.25 psi and 2.75 psi at hydrostatic line pressures of from 30 to 175 psi. Additionally, the static pressure drop across the first check valve must be at least 3.0 psi greater than the pressure differential required to open the relief valve. Furthermore, the complete backflow prevention device must not restrict flow in a manner such that pressure drops are greater than about 14 psi at capacity flow, with less than 10 psi pressure drop occurring across the first check valve at capacity flow and, preferably, between 6 and 10 psi at flows from zero flow to capacity flow.